"I expected a little bit of hazing. I didn't think they were going to hurt us," he told the outlet.
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The school said in a statement Friday that the shooting incident was unacceptable and a serious violation of its policies.
"New Mexico State University cares deeply about the health and welfare of our students. We work diligently to ensure a safe and secure environment where our students have an opportunity to succeed," the statement read. "We have taken steps to hold any responsible individuals accountable."
Attorney C.J. McElhinney, who is representing Altamirano, told NBC News in an email that his client thought the gun was unloaded and is "remorseful that Mr. Sillas was injured."
Altamirano was charged with felony aggravated assault, which McElhinney is contesting. He was also expelled from the university, his attorney said.
"The evidence in this case is consistent with the negligent handling of a firearm and not the more serious felony of aggravated assault that has been charged," he said. "It is not uncommon for the State of New Mexico to overcharge criminal defendants."
"I am confident that my client will be exonerated on the serious felony charge," McElhinney added.
The school was made aware of the shooting after being contacted by the Otero County Sheriff's Department.
The university's dean of students launched an investigation and during that inquiry learned that the fraternity was in violation of several of the school's code of conduct policies, according to documents from the school.
The alleged violations included providing alcohol to underage students at fraternity events and subjecting new members to a number of hazing incidents, including tackling the new members and pointing an unloaded gun at their heads and other body parts and pulling the trigger "as part of a loyalty activity."
The fraternity also held several events and activities that were not approved by the university, the documents state.
As a result of the violations, the university suspended Kappa Sigma until December 2024.
A spokesperson for the national Kappa Sigma fraternity said in a statement that it is "unequivocally opposed" to hazing, which violates the organization's code of conduct.
"We cannot comment on details of chapter or membership disciplinary proceedings, however, any member found to be in violation of the Fraternity’s Code of Conduct will be held accountable," the organization said.
Altamirano, 21, is scheduled for a preliminary hearing in February.
Minyvonne Burke is a breaking news reporter for NBC News.